lxd/backup: Call tar with --numeric-owner
lxd/backup: Call tar with --numeric-owner

Otherwise the encoded user/group names may differ on target system
leading to a rewrite of uid/gid...

Closes #6097

Signed-off-by: Stéphane Graber <089afc6d81f66f1168a9849e15660feae286e024@ubuntu.com>

Skarlso/lxd,atxwebs/lxd,stgraber/lxd,Skarlso/lxd,mjeanson/lxd,hallyn/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,Skarlso/lxd,lxc/lxd,hallyn/lxd,mjeanson/lxd,atxwebs/lxd,mjeanson/lxd,hallyn/lxd,hallyn/lxd,atxwebs/lxd,stgraber/lxd,lxc/lxd,atxwebs/lxd,lxc/lxd,lxc/lxd,hallyn/lxd,hallyn/lxd,lxc/lxd,Skarlso/lxd,stgraber/lxd,stgraber/lxd